Not sure whaich way is best for storing them , if they came rolled up , then I guess its okay to store them rolled up with the valve shut ,and more convenient.
If you can, the best way to store them is inflated with the valve open, i.e. just let the foam settle into its natural shape. No great harm in keeping them rolled, but they can take longer to re-inflate if the foam has been squashed up.
If you want to pack them small for travelling, it's usually easiest to roll loosely, close the valve and re-roll, opening the valve as you are almost done and then closing to seal.
You can store them rolled up and then when you come to use them, open the valve and leave it to inflate until you cannot hear the air anymore. Then close the valve and reroll the mat from the valve end to force the air down the far end and then Unroll and open the valve again to hear more air entering the valve.
This is a much quicker way to inflate these type of beds (particularly when its cold) and means you can store them fully deflated and rolled up.
for deflating open valves, fold mat in thirds and sit on it until most of the air is out, close valves then roll up opening the valves as you get the mat rolled up
I alway keep my mats inflated for storage as recommended by the manufacturer